508 



HENEY A. ROWLAND 



the segments prevent too great pressure on the screw. The final clamp- 

 ing is effected by the rings and screws, d, d, which enclose the flanges, e, 

 of the segments. The screw is now placed in a lathe and surrounded 

 by water whose temperature can be kept constant to 1 C., and the nut 

 placed on it. In order that the weight of the nut may not make the 

 ends too small, it must either be counterbalanced by weights hung from 

 a rope passing over pulleys in the ceiling, or the screw must be vertical 

 during the whole process. Emery and oil seem to be the only available 

 grinding materials, though a softer silica powder might be used towards 

 the end of the operation to clean off the emery and prevent future wear. 

 Now grind the screw in the nut, making the nut pass backwards and 

 forwards over the screw, its whole range being nearly 20 inches at first. 



FIG. 1. Section of Grinding Nut. 



Turn the nut end for end every ten minutes and continue for two weeks, 

 finally making the range of the nut only about 10 inches, using finer 

 washed emery and moving the lathe slower to avoid heating. Finish 

 with a fine silica powder or rouge. During the process, if the thread 

 becomes too blunt, recut the nut by a short tap so as not to change the 

 pitch at any point. This must, of course, not be done less than five 

 days before the finish. Now cut to the proper length; centre again in 

 the lathe under a microscope, and turn the bearings. A screw so ground 

 has less errors than from any other system of mounting. The periodic 

 error especially will be too small to be discoverefl, though the mountings 

 and graduation and centering of the head will introduce it; it must 

 therefore finally be corrected. 



Mounting of Screws. The mounting must be devised most carefully, 

 and is, indeed, more difficult to make without error than the screw itself. 

 The principle which should be adopted is that no workmanship is per- 

 fect; the design must make up for its imperfections. Thus the screw 



